Aluminum-based plated steel sheet and its manufacturing method

Abstract

The present invention relates to an aluminum-plated steel sheet used in automobiles and the like, and a method for producing the same.

Description

[Technical Field]

[0001] The present invention relates to an aluminum-plated steel sheet used in automobiles and the like, and a method for producing the same. [Background technology]

[0002] Recently, efforts have been made to reduce the weight of automobiles to improve fuel efficiency. To achieve this, the thickness of steel materials can be reduced, but this reduction can cause problems with the stability of automobiles, so the strength of the steel must be improved. For this reason, there is a continuous demand for high-strength steel materials, and various types of steel materials have been developed. However, due to their high strength, these steel materials have problems with poor workability, such as springback.

[0003] To solve these problems, a hot press forming process has been proposed. This hot forming process is also called hot press forming or hot working. The hot forming process involves processing a steel material at a high temperature (800°C or higher) where it is easy to process, and then press-forming it in a mold while rapidly cooling it to a low temperature, thereby forming a low-temperature structure such as martensite in the steel material and increasing the strength of the final product. This can minimize workability issues when manufacturing high-strength components.

[0004] Hot forming has the advantage of not only making it possible to easily form complex shapes but also ensuring high strength for the manufactured parts (hot-formed components), and therefore its use has been increasing recently. In particular, corrosion resistance is ensured by using plated steel material with aluminum, zinc, etc. on the surface of the steel material. As an example, Patent Document 1 discloses the use of aluminum-plated steel sheet in the hot forming process.

[0005] When hot forming is performed using the above aluminum-plated steel sheet, there is a problem that the corrosion resistance is weak due to the lack of sacrificial corrosion protection of aluminum. Since the client company requires aluminum-plated steel sheet with high corrosion resistance, they are developing new plated steel sheet by adding elements such as Zn and Mg during the aluminum plating.

[0006] However, existing auto parts manufacturers use hot forming and have hot forming processes optimized for existing aluminum-plated steel sheets, so there are limitations to applying the hot forming process to new aluminum-plated steel sheets containing elements such as Zn and Mg.

[0007] Therefore, it is necessary to utilize existing aluminum-plated steel sheets. Because aluminum-plated steel sheets usually contain a small amount of silicon, they are sometimes called AlSi-plated steel sheets. Because AlSi-plated steel sheets are alloy-plated steel sheets of aluminum and silicon, they generally consist of an aluminum structure and a silicon structure. In particular, as the plated steel sheet cools, the silicon solidifies in the form of a separate precipitate, resulting in the formation of spangles, a surface characteristic of AlSi-plated steel sheets.

[0008] When spangles are formed on the surface of the aluminum-based plated steel sheet, the surface becomes less attractive and has a negative effect on corrosion resistance. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Patent No. 6,296,805 Summary of the Invention [Problem to be solved by the invention]

[0010] One aspect of the present invention relates to an aluminum-based plated steel sheet, and more particularly to an aluminum-based plated steel sheet having excellent surface quality and corrosion resistance, and a method for producing the same.

[0011] The object of the present invention is not limited to the above-mentioned content, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention from the overall matters of the specification of the present invention. [Means for solving the problem]

[0012] One aspect of the present invention relates to an aluminum-based plated steel sheet comprising a base steel sheet and an Al plating layer formed on the base steel sheet, wherein the size of spangles on the surface of the Al plating layer after surface etching is 2.5 mm or less.

[0013] After surface etching of the plated steel sheet, the fraction of the Si phase observed on an optical image can be 40 area % or less.

[0014] The aluminum-based plated steel sheet may have a whiteness of 85% or more.

[0015] The aluminum-based plated steel sheet may have a gloss of 60 GU or more.

[0016] The above base steel sheet contains, in wt.%, C: 0.02-0.6%, Si: 0.001-2%, Al: 0.001-1%, Mn: 0.1-4%, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, Ti: 0-0.1%, B: 0.0001-0.01%, Cu: 0-1.00%, Mo: 0-1.00%, Cr: 0-1. 0.00%, Ni: 0-1.00%, V: 0-1.00%, Ca: 0-0.01%, Nb: 0-0.1%, Sn: 0-1%, W: 0-1%, Sb: 0-1%, Mg: 0-0.1%, Co: 0-1%, As: 0-1%, Zr: 0-1%, Bi: 0-1%, REM: 0-0.3%, the remainder may contain Fe and impurities.

[0017] Another aspect of the present invention is a method for manufacturing a steel sheet, the method comprising the steps of: immersing a base steel sheet in an aluminum (Al) plating bath containing silicon (Si) to adhere a plating solution to a surface of the base steel sheet; adjusting the coating weight on the surface of the base steel sheet using an air knife (A / K); and The present invention relates to a method for producing an aluminum-based plated steel sheet, which includes a step of transferring a base steel sheet having a plating solution attached thereto to a cooling means and cooling the same, and which satisfies the K value defined by the following formula 1 of 200 to 400.

[0018] (Formula 1)K=10 6 *(a*d) / (b*c*e)

[0019] where a: Si content of the plating bath (wt.%), b: line speed (mpm), c: A / K spacing (mm), d: A / K pressure (kPa), e: A / K height (mm). [Effects of the Invention]

[0020] The aluminum-based plated steel sheet, which is an example of the present invention, can provide an aluminum-based plated steel sheet that has a beautiful surface and excellent corrosion resistance.

[0021] The various beneficial advantages and effects of the present invention are not limited to the above, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0022] [Figure 1] 10A and 10B are photographs of the surface of the plated steel sheet of Comparative Example 8 before and after etching, respectively. [Figure 2] 1 shows the results of EPMA (Electron Probe X-ray Micro Analyzer) analysis of Comparative Example 8. [Figure 3] Photographs (a) to (c) show the surfaces of invention examples 1 and 3 and comparative example 8 before and after etching, respectively. [Figure 4] 1(a) to 1(c) are optical images and image analysis results of invention examples 1 and 3 and comparative example 8, respectively. DETAILED DESCRIPTION OF THE INVENTION

[0023] The terminology used herein is for the purpose of describing the invention and is not intended to limit the invention. Also, as used herein, the singular forms "a," "an," and "the" include the plural forms unless the related definition clearly dictates otherwise.

[0024] The meaning of "comprises" as used in the specification is to specify features and does not exclude the presence or addition of other features.

[0025] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Dictionary-defined terms are to be interpreted as having a meaning consistent with the relevant technical literature and the presently disclosed content.

[0026] Spangles that form on the surface of aluminum-based coated steel sheets, i.e., AlSi-coated steel sheets, are a phenomenon that occurs when silicon is separately precipitated during cooling of the coated steel sheet. In other words, needle-like silicon structures are formed on the surface of the coated steel sheet during the cooling process, and these needle-like silicon structures cause the spangle phenomenon (see Figures 1 and 2). Spangles prevent the coated steel sheet from achieving a beautiful surface and negatively affect corrosion resistance by causing galvanic corrosion between the aluminum and silicon adjacent to the spangles.

[0027] Various methods are used to control spangles, among which the addition of other elements to the plating bath during plating is widely used. However, there are many difficulties in controlling the plating bath, so there is a need for a technology that can control the size of spangles without changing the special additional components in the plating bath.

[0028] Hereinafter, an aluminum-based plated steel sheet according to one embodiment of the present invention will be described in detail.

[0029] An aluminum-based plated steel sheet according to one embodiment of the present invention includes a base steel sheet and an Al plating layer formed on the base steel sheet, The size of the spangles on the surface of the Al plating layer can be 2.5 mm or less when visually inspected after surface etching.

[0030] The Al coating layer of the aluminum-based coated steel sheet is an alloy of Al and Si, and therefore generally contains both Al and Si structures. Si solidifies as a separate precipitate during cooling of the coated steel sheet, which can result in the spangle phenomenon, a surface feature of the coated steel sheet. In this case, Si and Al are not mixed in the spangle region on the surface of the coated steel sheet, but rather aggregate with each other, resulting in galvanic corrosion at the interface between Si and Al, which can adversely affect corrosion resistance. Furthermore, if the spangles are large and noticeable on the surface, they can adversely affect the aesthetics of the coated steel sheet surface. Therefore, the spangle size is preferably 2.5 mm or less when visually inspected after surface etching. Meanwhile, the shape of the spangles observed after etching is a diamond, consisting of two triangles joined together. The length of the long axis of the diamond can be defined as the size of one spangle.

[0031] The surface etching is not particularly limited, but as a specific example, etching can be performed with 4 g of NaCl and 200 mL (40% H3PO4) at room temperature for 1 to 3 minutes.

[0032] The plated steel sheet may have a whiteness of 85% or more and a gloss of 60GU or more. Because the plated steel sheet has a whiteness of 85% or more and a gloss of 60GU, a beautiful surface can be ensured.

[0033] Fig. 1 is a photograph of the surface of the plated steel sheet of Comparative Example 8 before etching (a) and after etching (b), and Fig. 2 is the result of an EPMA (Electron Probe X-ray Micro Analyzer) analysis of Comparative Example 8. Hereinafter, one example of the present invention will be further described with reference to Figs. 1 and 2.

[0034] The spangles are a phenomenon that occurs when Si precipitates separately during cooling of the plated steel sheet, and when the surface of the plated steel sheet is observed with an EPMA (Electron Probe X-ray Micro Analyzer), they are sometimes found to be needle-like Si structures. Figures 1 and 2 are photographs of the surface of Comparative Example 8 of the following Examples.

[0035] On the other hand, the fraction of the Si phase on the optical image of the surface of the Al plating layer after the etching may be 40 area % or less. If the Si phase on the surface is too much, galvanic corrosion may occur between the Al phase and the Si phase on the surface, which may reduce corrosion resistance.

[0036] There are no particular restrictions on the base steel sheet, as long as it can be used for hot forming. There are no particular restrictions on manufacturing process classifications such as hot-rolled steel sheet and cold-rolled steel sheet, as well as on steel types and alloy composition systems such as dual phase steel (DP), multi-phase steel, TRIP steel, and TWIP steel.

[0037] A typical example is 22MnB5 steel, and the specific contents may be, in wt. % (weight %), carbon (C): 0.1 to 0.3%, manganese (Mn): 1.0 to 2.0%, silicon (Si): 0.02 to 0.30%, boron (B): 5 to 45 ppm, with the remainder being unavoidable impurities and iron (Fe).

[0038] The chemical composition of the base steel sheet is, in wt.%, carbon (C): 0.02 to 0.6%, silicon (Si): 0.001 to 2%, aluminum (Al): 0.001 to 1%, manganese (Mn): 0.1 to 4%, phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, titanium (Ti): 0 to 0.1%, boron (B): 0.0001 to 0.01%, copper (Cu): 0 to 1.00%, molybdenum (Mo): 0 to 1.00%, chromium (Cr): 0 to 1.00%, nickel (Ni): 0.0001 to 0.01%, and chromium (Cr): 0.0001 to 0.01%. The steel may be composed of nickel (Ni): 0-1.00%, vanadium (V): 0-1.00%, calcium (Ca): 0-0.01%, niobium (Nb): 0-0.1%, tin (Sn): 0-1%, tungsten (W): 0-1%, antimony (Sb): 0-1%, magnesium (Mg): 0-0.1%, cobalt (Co): 0-1%, arsenic (As): 0-1%, zirconium (Zr): 0-1%, bismuth (Bi): 0-1%, rare earth metals (REM): 0-0.3%, with the remainder being Fe and impurities.

[0039] Next, an embodiment of a method for producing an aluminum-plated steel sheet according to the present invention will be described in detail.

[0040] The method for producing the aluminum-plated steel sheet includes the steps of immersing a base steel sheet in a coating bath to deposit a coating solution on the surface of the base steel sheet, adjusting the coating weight on the surface of the base steel sheet, and transferring the base steel sheet with the coating solution deposited thereon to a cooling means. Each step will be described in detail below.

[0041] First, a base steel sheet is immersed in a coating bath to allow the coating solution to adhere to the surface of the base steel sheet. Specifically, the base steel sheet is immersed in an aluminum (Al) coating bath containing silicon (Si) to allow the coating solution to adhere to the surface of the base steel sheet.

[0042] The temperature at which the base steel sheet is drawn into the coating bath is effectively 620 to 680°C. Before immersing the base steel sheet into the coating bath, the base steel sheet can be heated to a certain temperature (drawing temperature). In this case, the drawing temperature is effectively 620 to 680°C. By heating the base steel sheet to the drawing temperature, problems such as rapid solidification of molten aluminum on the surface of the steel sheet, resulting in reduced fluidity and resulting in uneven coating weight or uncoated areas, can be prevented. However, excessive heating may actually promote the dissolution of the steel sheet, accelerating the generation of dross.

[0043] The coating bath may contain silicon (Si): 6 wt.% or more but less than 11.5 wt.%, iron (Fe): 1 to 4 wt.%, and the remainder being aluminum (Al) and inevitable impurities. The coating bath may be an Al-based coating bath, and the Al-based coating bath is for forming an Al coating layer. As long as the coating is applicable to coated steel sheets for hot forming, it can be applied to the present invention without any restrictions. As a preferred example, the composition of the coating bath may contain silicon (Si): 6 wt.% or more but less than 11.5 wt.%, iron (Fe): 1 to 4 wt.%, and the remainder being aluminum (Al) and inevitable impurities.

[0044] As the Si content in the coating bath increases, the melting point of the coating bath decreases, which is thought to make it easier to cool the coated steel sheet, and although the higher the Si content, the more beautiful the surface of the coated steel sheet can be obtained, in reality, the coated steel sheet obtained has a deteriorated surface. This is because when the Si content exceeds 11.5 wt.%, it is near the eutectic point in the Al-Si binary phase, and a large amount of Si aggregates and solidifies at once, increasing the size of the spangles, resulting in a coated steel sheet with a deteriorated surface.

[0045] The temperature of the plating bath is effectively 630 to 680° C. If the temperature of the plating bath is too low, the fluidity of the plating solution in the plating bath may decrease, whereas if the temperature is too high, the generation of dross in the plating bath may increase.

[0046] Next, the coating weight of the base steel sheet is adjusted. The means or method for adjusting the coating weight is not particularly limited, and can be a method commonly used in the technical field to which the present invention pertains. For example, an air knife (A / K) is used.

[0047] The above plating coverage is 8 to 80 g / m on one side 2 The effective thickness of the plating layer is 3 to 30 μm. The plating weight of Al plating can usually be converted to the plating weight when the plating layer thickness is multiplied by 2.7, and when a plating layer thickness of 3 to 30 μm is converted to the plating weight, it is about 8 to 80 g / m 2 The more preferable thickness is 4 to 26 μm, and the plating weight is 10 to 70 g / m 2 is.

[0048] The base steel sheet with the coating weight adjusted is transferred to a cooling means for cooling. When the base steel sheet with the coating solution adhered thereto enters the cooling means, rapid cooling begins, which can stop the reaction of the alloy layer in the Al coating layer. One example of the cooling means is a cooling tower.

[0049] In the above production method, it is effective that the K value defined by the following formula 1 is 200 to 400.

[0050] The above equation 1 can be calculated as follows:

[0051] (Formula 1)K=10 6 *(a*d) / (b*c*e)

[0052] where a: Si content of the coating bath (wt.%), b: line speed (mpm), c: A / K spacing (mm), d: A / K pressure (kPa), and e: A / K height (mm). Line speed refers to the speed at which the steel sheet is transported through the coating bath, the air knife spacing (A / K) for adjusting the coating weight refers to the spacing between the steel sheet and the air knife, and the A / K height refers to the distance from the coating bath to the A / K.

[0053] The above formula (1) takes into consideration the respective characteristics of various variables that determine the properties of the coating layer in order to produce a coated steel sheet with a beautiful surface, and has technical significance in that it takes into consideration factors that affect the production of a coated steel sheet and their correlations for the purpose of producing a beautiful surface.

[0054] In order to produce coated steel sheets with beautiful surfaces, various process factors such as the Si content of the coating bath, coating bath temperature, drawing temperature, line speed, A / K interval, A / K pressure, A / K height, and cooling rate of the cooling tower must be considered, and the Si content, line speed, A / K interval, A / K pressure, and A / K height of the coating bath must be optimized. This is derived using the above (Equation 1). When the K value in the above (Equation 1) is 200 to 400, coated steel sheets with beautiful surfaces can be obtained.

[0055] Specifically, when the K value is less than 200, the A / K pressure is not high enough to reduce the spangle size, or the line speed, A / K spacing, and A / K height are low, making it difficult to reduce the spangle size. When the K value is more than 400, the A / K pressure is too high, or the line speed, A / K spacing, and A / K height are higher than appropriate, making it difficult to reduce the spangle size by cooling the coated steel sheet.

[0056] The base steel sheet may be prepared in various ways before being immersed in a coating bath. For example, the base steel sheet may be manufactured by heating a steel slab, followed by hot rolling, coiling, cold rolling, annealing, etc. The specific process conditions required for the heating, hot rolling, coiling, cold rolling, annealing, etc. of the steel slab are not particularly limited, as they may vary depending on the properties required of the base steel sheet. [Example]

[0057] Examples of the present invention will now be described. It goes without saying that various modifications of the following examples are possible within the scope of the present invention, provided that they are understood by those skilled in the art. The following examples are provided for the purpose of understanding the present invention, and the scope of the present invention should not be limited to the following examples, but should be defined by the claims below as well as equivalents thereof.

[0058] (Example) In the examples of the present invention, a conventional 22MnB5 base steel sheet was prepared and immersed in a coating bath containing approximately 7.5 to 11.5 wt.% Si, with the remainder being Al and unavoidable impurities, to produce an aluminum-coated steel sheet. The temperature of the coating bath was 640 to 670°C, and the drawing temperature of the base steel sheet was 650 to 680°C.

[0059] In order to produce the above-mentioned plated steel sheet, the Si content in the plating bath, the transport speed (line speed) of the steel sheet, and the thickness of the plating bath are set to 10 to 80 g / m 2 The coating weight on one side was adjusted using an air knife (A / K) and then transferred to a cooling tower for cooling. The process conditions are shown in Table 1.

[0060] [Table 1]

[0061] The above equation 1 can be calculated as follows:

[0062] (Formula 1)K=10 6 *(a*d) / (b*c*e)

[0063] where a: Si content of the plating bath (wt.%), b: line speed (mpm), c: A / K spacing (mm), d: A / K pressure (kPa), e: A / K height (mm).

[0064] The test pieces produced as described above were checked for the size of the spangles after surface etching, and the whiteness, gloss and surface beauty of each test piece were checked. The results are shown in Table 2 below.

[0065] The whiteness and gloss values ​​shown above are the average values ​​of the measurements taken three times on each side of the front and back of each test piece after cutting them into 150 x 150 mm pieces, with whiteness being the L value of the measurements and gloss being the value at 60 degrees. Perfect whiteness is considered to be 100 for whiteness, and the degree of whiteness is expressed as a percentage (%), while gloss is measured in GU (Gloss Unit).

[0066] The surface etching was carried out for 1 to 3 minutes using a solution of 4 g of NaCl and 200 mL (40% H3PO4).

[0067] The spangle size was measured by taking a photograph of a 50 x 50 mm plated steel sheet and analyzing the photograph. The shape of the spangles observed after etching was a diamond, consisting of two triangles joined together. The length of the long axis of the diamond was defined as the size of one spangle, and the average length of the long axes of 25 such diamonds was defined as the spangle size during image analysis. In particular, for ease of measurement and clarity of standards, the average of the long axes of 25 large diamonds was measured and used as the spangle size.

[0068] On the other hand, in Table 2 below, when the size of the spangles was so small that it was difficult to visually confirm them, it was indicated as "not detected."

[0069] The judgment of whether the surface was beautiful or not was made by describing the surface form of the aluminum-plated steel sheet as "normal," describing a case where the spangles were too large and their shapes were conspicuous as "inferior," and describing a case where the surface was more beautiful than the surface of a normal aluminum-plated steel sheet as "excellent."

[0070] [Table 2]

[0071] As can be seen from the results in Tables 1 and 2 and FIGS. 1 to 4 above, aluminum-plated steel sheets corresponding to the examples of the invention that satisfy the ranges set forth in the present invention either have no spangles observed on the surface after etching or have a size of 2.5 mm or less, and at this time have a whiteness of 85% or more and a gloss of 60 GU or more, thereby ensuring excellent surface properties.

[0072] 1(a) and 1(b) are photographs of the surface of Comparative Example 8 before and after etching, and FIG. 2 shows the results of EPMA analysis of Comparative Example 8.

[0073] FIG. 1(a) is a photographic image of Comparative Example 8, which shows that a pattern was formed on the surface of the plated steel sheet. To further confirm this, FIG. 1(b) shows a photographic image of an AlSi plated steel sheet whose surface was etched.

[0074] The spangle pattern on the AlSi-plated steel sheet is a phenomenon that appears when Si is separately precipitated during cooling of the plated steel sheet. FIG. 2 shows the results of observing the surface of the plated steel sheet of Comparative Example 8 using an EPMA (Electron Probe X-ray Micro Analyzer), and it can be seen that the spangle pattern is caused by needle-like Si.

[0075] Figure 3(a) to (c) are photographs of the surfaces of Examples 1 and 3 and Comparative Example 8 before and after etching, respectively. Figure 4(a) to (c) are optical images and image analysis results of Examples 1 and 3 and Comparative Example 8, respectively. From the results of Figures 3 and 4, it can be seen that the surface Si phase fraction of the Examples was reduced to 40 area % or less, and as the size of the Si phase is reduced as described above, it is expected that this may have a further impact on future improvements in corrosion resistance.

Claims

1. The steel sheet includes a base steel sheet and an Al plating layer formed on the base steel sheet, an aluminum-based plated steel sheet, wherein the size of spangles on the surface of the Al plating layer is 2.5 mm or less after surface etching.

2. 2. The aluminum-based plated steel sheet according to claim 1, wherein the fraction of the Si phase observed on an optical image after surface etching of the plated steel sheet is 40 area % or less.

3. 2. The aluminum-based plated steel sheet according to claim 1, wherein the aluminum-based plated steel sheet has a whiteness of 85% or more.

4. The aluminum-based plated steel sheet according to claim 1, wherein the aluminum-based plated steel sheet has a gloss of 60 GU or more.

5. The base steel sheet contains, in wt. %, C: 0.02 to 0.6%, Si: 0.001 to 2%, Al: 0.001 to 1%, Mn: 0.1 to 4%, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, Ti: 0 to 0.1%, B: 0.0001 to 0.01%, Cu: 0 to 1.00%, Mo: 0 to 1.00%, Cr: 0 to 1.00%, Ni: 0 to 1.00%.

2. The aluminum-based plated steel sheet according to claim 1, wherein the aluminum-based plated steel sheet comprises 0% by weight, 0.0% by weight, V: 0 to 1.00%, Ca: 0 to 0.01%, Nb: 0 to 0.1%, Sn: 0 to 1%, W: 0 to 1%, Sb: 0 to 1%, Mg: 0 to 0.1%, Co: 0 to 1%, As: 0 to 1%, Zr: 0 to 1%, Bi: 0 to 1%, REM: 0 to 0.3%, and the balance being Fe and impurities.

Images